Based on the results of that trial, a two-shot version of the vaccine -- which includes a "primer" that jumpstarts the immune system before the Ebola vaccine is given -- is now being tested for the first time in humans, said Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases.

"This is the animal study version of that vaccine," Fauci said of the monkey trial. "This vaccine looked very good in animal studies. It protected monkeys from a lethal challenge of Ebola."

Fauci said the human trial is being done to find out if the vaccine is safe and if the people who get the vaccine create antibodies to the virus.

Two women were given the experimental vaccine last week as the U.S. National Institutes of Health launched a much-anticipated trial to combat the often-lethal Ebola virus that has plagued four West African nations. The women, ages 39 and 27, were the first people to receive the vaccine. The human trial will eventually include 20 men and women ages 18 to 50. No one will be infected with the disease. The vaccine was developed by the U.S. National Institute of Allergy and Infectious Diseases and drug maker GlaxoSmithKline.

Fauci, describing the human trial, said: "This will take us, at the minimum, to the end of the year. And if it is safe and does have a response that you would predict would be protective, then you would move on to the next phase to study it over a year to find out what the right dose is, what the response is and the long-term safety."

But given the current outbreak plaguing West Africa that has sickened more than 3,000 people and killed more than 1,900, the process may be speeded up, Fauci said. "Once safety is established, one would likely do a clinical trial to determine if it works and is protective," he said.

The people most likely to be given the vaccine would be those at the highest risk in West Africa, Fauci said, "the health care workers who put themselves in harm's way when they take care of people."

Fauci cautioned, however, that this or any other vaccine won't control the current Ebola epidemic in West Africa.

"We are in the very early stages of intervention with vaccines and drugs. The mainstay of controlling this epidemic is to intensify infection control," he stressed.

Although this is the first vaccine to reach human trials, Fauci said human testing of other vaccines will start in about a month.

The report on the monkey trial was published online Sept. 7 in the journal Nature Medicine.

In the trial, a team led by Nancy Sullivan, chief of the biodefense research section at the U.S. National Institute of Allergy and Infectious Diseases, kick-started the monkeys' immune systems by first giving them an injection of a virus to which the animals had already been exposed and then giving them the Ebola virus. This two-shot approach provided immunity for up to 10 months, the researchers said.

The researchers also tried a single-shot version of the vaccine, but found that it only worked for about five weeks.

Thomas Geisbert, a professor of microbiology and immunology at the University of Texas Medical Branch at Galveston, said that eventually having a single-shot vaccine is important.

The successful approach used in the monkey trial, where the immune system was "primed" to better respond to Ebola is interesting, he said. However, it is doubtful that that approach would work in an Ebola outbreak, he said.

"In an outbreak, you would not have time to prime and boost," Geisbert said. "You really need a fast-acting single-injection vaccine that you can give to health care workers and first responders right before you send them into the hot zone," he said.

Once a vaccine is available, not everyone in areas where Ebola is endemic should be vaccinated, Fauci said.

"If you are in a region where there is a high risk for an outbreak, you would start with the first responders and emergency room personnel. If the outbreak persists, you might consider vaccinating a larger number of people," he explained.

Dr. Amesh Adalja, an associate at the University of Pittsburgh's Center for Biosecurity, called the experimental vaccine "promising."

Adalja said there are many vaccines for Ebola in the works. "Any of these vaccines could change the face of how we deal with Ebola outbreaks in the future," he said.

But, he added, "the outbreak in West Africa will be stopped using the tried-and-true methods used in the past. It would be a mistake for people to believe that an experimental Ebola vaccine is going to have a meaningful impact on this outbreak."

U.S. and World Health Organization officials warned last week that the highly virulent disease is spreading faster than health workers in Guinea, Liberia, Nigeria and Sierra Leone can work to contain it. WHO officials said as many as 20,000 people could become infected with the virus, which has a mortality rate that can approach 90 percent in some cases.